This invention relates to soft x-ray optics and methods for reflection and dispersion of x-rays having wavelengths within the range of 2.5 .ANG. to 124 .ANG. (100 eV to 5000 eV). More particularly, it relates to the preparation and use of improved x-ray mirrors and dispersion elements operable within this wavelength range in x-ray spectrometers and imaging and non-imaging systems.
The use of x-rays as an analytical tool is currently being applied in many fields of technology. X-ray spectroscopy, for example, is a technique for elemental analysis. Other applications of x-ray analysis arise because x-rays exhibit properties similar to those of light waves generally. Accordingly, x-ray optics have been applied to develop imaging applications in astronomy, biology, medical research, microlithography, thermonuclear fusion, synchrotron radiation and related fields. Numerous devices have been developed for implementing such applications and generally are only as accurate as the optical elements that reflect, refract, or diffract the radiation.
Two applications which are of particular interest in the wavelength region considered here are "water window" x-ray microscopy and x-ray lithography. The water window is a region of the spectrum between the carbon and oxygen K edges, (the K edge is the x-ray energy below which x-rays are not strongly absorbed because they cannot generate a transition of the K electron in the element of interest). Because carbon absorbs much more strongly than oxygen in this region, a high quality optical system designed to work within this range could obtain high contrast images of still living cells in their natural, aqueous environment. This possibility is of such interest that it has long been a focus of research. Many systems have been designed, built and operated with some success despite the poor performance of previous reflective devices. One of the first multilayer systems studied was comprised of alternating layers of vanadium and carbon, V/C, and was intended for operation in the water window region of the spectrum. Many other material pairs have been studied over the past twenty years, in an effort to improve performance in this region, but without success.
Previous x-ray lithography work has focused on 130 .ANG. radiation. Mo/Si multilayers have been developed to achieve very high reflectivity at this wavelength. However, recently, it has become apparent that other considerations will require a working x-ray lithography system to operate at smaller wavelengths. Unfortunately, multilayers designed to work at the desired wavelengths have exhibited relatively poor performance.
Multilayer optics, as developed by Spiller, as discussed in the article Revolution in X-Ray Optics, Journal of X-Ray Science and Technology, 1, 7-78 (1989), can be manufactured using multiple stacked combinations of bi-layered structure having a very small period (10 .ANG. to 200 .ANG.). The bi-layer consists of one layer of material having low absorption and a second layer which operates as a spacer in integral contact with the first layer. The index of refraction of the second or spacer layer is as different from the first as possible. These layers are stacked in multiples to develop an effective reflectivity device which operates approximately according to Bragg's law: n.lambda.=2 d sin .theta..
The technology to make these structures usually involves sputtering (or some other form of vapor deposition) a material of the desired composition onto a substrate in the form of a thin layer. As used herein, vapor deposition refers to processes by which a bulk deposit is formed on a substrate as individual atoms or molecules arrive at its surface, building it up. As used herein, thin layer means a layer which is sufficiently thin to be optically responsive to incident x-ray radiation by causing reflection, refraction or diffraction. As used herein, reflection, refraction or diffraction will all be identified by the term reflection. For x-ray wavelengths within the range of 2.5 to 125 Angstroms, typical layer thicknesses are from a few Angstroms to over 100 Angstroms. While most of this range may be achieved through standard vapor deposition techniques including sputtering, evaporation and chemical vapor deposition, layers of only a few Angstroms thickness are more difficult to produce. Specialized techniques are required in order to achieve such ultra thin layers of sufficient quality for multilayer optics.
The peak reflectivity that these multilayered, thin films can achieve is primarily influenced by the optical constants, which can be numerically represented by index of refraction and index of absorption values. The value of the appropriate x-ray optical constants of materials is crudely determined by the electron density of the solid films and by electronic transition energy levels within the atoms. As a general rule, heavier elements have relatively high indices of both refraction and absorption, and lighter elements have relatively low indices of refraction and absorption.
In order to obtain high reflection from a multilayered optical device, the alternating layers are selected to maximize the difference in the complex index of refraction (n=1-.delta.-i.beta.) between materials comprising the respective layers of the bi-layers while minimizing the absorption. The material with n most different from 1 is often referred to as the absorbing layer. The other layer of the bi-layer is known as the spacer layer.
It is well known that the highest reflecting multilayers use relatively heavy metals or mixtures of heavy metals as the absorbing layer. Such elements as molybdenum, tungsten, iron, nickel, tantalum, vanadium, rhenium, or ruthenium have been used for the absorbing layer. Elements with high atomic number tend to have larger indices of refraction (.delta.), especially when their density is also high. However, these elements also tend to have higher indices of absorption (.beta.), which degrades multilayer performance. This creates a trade off which makes material selection more difficult. Experience has shown that at most wavelengths within the soft x-ray region best absorbing materials tend to fall into one of three groups. These groups are:
V-Ni group consisting of V, Cr, Fe, Co and Ni
Zr-Rh group consisting of Zr, Nb, Mo, Ru and Rh
Hf-Os group consisting of Hf, Ta, W, Re and Os.
These groups correspond to portions of the 3d, 4d and 5d transition metal groups in the periodic table, respectively. They are also characterized by their refractory and high density similarities.
Theoretical performance at a given wavelength is often remarkably similar within a group, and the best group depends on the wavelength of interest. It has long been known that the best group over much of the 100 eV to 500 eV x-ray range is the V-Ni group. This accounts for the early attempts to manufacture V/C multilayers, and the extensive recent work by many researchers using every other element in this group with the possible exception of Co.
Unfortunately, the V-Ni group has the lowest atomic numbers of the three. Because the atomic number is lower, both the absorption and the index of refraction are relatively small. The theoretical superiority of this group is due more to uncharacteristically poor performance for the other groups rather than good performance for the V-Ni group. Thus, even the theoretical performance of the best existing multilayers for this region of interest is low when compared to the theoretical performance of the best multilayers in other regions of spectrum.
The spacer layer also poses unusual difficulties over much of the wavelength range of interest. Commonly used spacer materials are carbon, boron, and silicon. However, while carbon and boron are good choices over some of the range considered here, above about 290 eV both have relatively high absorption. Therefore, other materials, with low absorptance, such as scandium, titanium, sodium, calcium, and lithium are required.
The discussion above has considered only the optical properties of the materials in question, but in actual multilayers other considerations are as important. In order to obtain performance approaching the theoretical values obtained from the optical constants, it must be possible to deposit both materials in thin, alternating layers. Good control over thickness and extremely smooth, abrupt interfaces are required. The usual approach to this problem is to attempt to deposit amorphous layers, or if that is not possible to obtain polycrystalline layers with extremely small grain sizes. Another more difficult, but potentially very rewarding approach, is to deposit oriented crystalline materials epitaxially layer by layer. In any case, the materials should also be refractory and chemically stable, as reactions or diffusion at the layer boundary can smear the boundary and significantly degrade performance. Also, reactions with atmospheric gasses and other common airborne contaminants such as sulfur can destroy the layer structure. These considerations can greatly restrict the possible materials.
Lithium, sodium, magnesium, potassium, and calcium would all make good low index materials at many wavelengths, based on their optical properties, but they have never been successfully used due to their high reactivity. Most of the elements in the V-Ni group have problems in this regard. Though some of them are not overly reactive, they are perhaps not refractory enough and are difficult to deposit in smooth layers. The most commonly and successfully used high index materials are tungsten and molybdenum. Both of these materials are highly refractory, with melting temperatures of 3410.degree. C. and 2610.degree. C. respectively. This quality cannot be over emphasized. Smoothness and resistance to layer interdiffusion is so critical and so difficult to obtain that no commercially successful multilayers have been developed using an absorbing material with a melting point below 1400.degree. C.
The standard approach to multilayer design is to select the spacer material first. This is done because the system tends to be more sensitive to changes in the spacer material. Then the absorbing material is selected to be compatible with the spacer material. Usually the absorbing material is selected from the V-Ni group, the Zr-Rh group, or the Hf-Os group. When a material outside of these groups is selected the choice is almost always based on superior material properties, especially in combination with the already chosen spacer material. However, the results of experiments which used elements outside of these groups have never yielded results as good as those obtained by using elements within the groups.
The invention disclosed herein departs from long-established selection criteria as discussed above and identifies uranium composition as having surprising utility. By choosing uranium, not only have we have gone outside of the three preferred groups but we have also chosen an element without the normal good material properties to recommend it. Uranium belongs to the actinide series of elements which more closely resembles the lanthanide or rare earth elements in their chemical and physical properties. Uranium and the lanthanides tend to be quite chemically reactive and relatively low melting, unlike the transition metals usually selected. To our knowledge, the rare earth elements have never been used in commercially available soft x-ray optics.
Due to the fact that uranium is highly reactive and will burn in air at temperatures not greatly in excess of room temperature it has not previously been considered for use in multilayers. It forms oxides immediately in air which causes rapid disintegration of uranium thin film structures. Thus any multilayer containing uranium must be both carefully made and well protected from exposure to the normal environment.
Uranium's melting point is only a moderate 1132.degree. C., as opposed to &gt;&gt;2000.degree. C. for the most successful high index materials. Moreover, its atomic density is also only moderate. The mass density of uranium is less than that of tungsten, rhenium, osmium, iridium, platinum and gold, despite the much higher atomic weight of uranium.
Recent information in the literature indicates that the optical properties of uranium may be promising in the wavelength range of interest, however, we found that fabrication of uranium containing multilayers with adequate layer structure is possible, though not trivial. Actual measurements of fabricated uranium containing multilayers have shown that, contrary to prior art expectation based on the body of evidence and experience, properly designed and manufactured uranium containing multilayers can be superior to those constructed from the V-Ni group typically used for this wavelength range.